The gathering, which was held at Downtown Mayfair, was a sure indicator that the Middleton family members have been warmly welcomed into the royal fold. James, 25, rivalled his elder sisters Kate and Pippa Middleton in the style stakes by wearing a dapper navy blue suit and a crisp white shirt.

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It seems that the two princesses and their mother the Duchess of York, who were all dressed in varying monochrome shades, might have taken style inspiration from the expectant Duchess of Cambridge who had sported the trend earlier in the day at a charity visit.

Beatrice left Italian restaurant hand in hand with her mum. Her sister Eugenie, 22, who is studying at Newcastle University, left with her bartender boyfriend Jack Brookbanks.

Birthday girl Heather was considerably more colourful than the royals in a floor length fuschia pink gown. She invited an eclectic mix of guests to the party including Monica Lewinsky, Donna Air, Trinny Woodall and Rolling Stone Ronnie Wood and his theatre producer wife Sally Wood.

Downtown Mayfair, which serves Northern Italian and Japanese cuisine, is a sister restaurant to C London restaurant (formerly the Cipriani) and the iconic Harry's Bar in Venice.